The invention relates to a drive pulley for elevators and a method of manufacture thereof.
As is known, drive pulleys for elevators are currently made from cast iron (ÖV 200 or ÖV 250) containing lamellar graphite. Hematic grey raw cast iron with an addition of ferrosilicon is usually used as the manufacturing base material for that purpose. After casting is concluded, i.e. after cooling of the casting, a stress-relieving treatment is carried out. Thereafter the drive pulley from the casting is processed to a finished state by stress relief (see, for example, the book by Dr. Verö and Dr. Káldor, Metallurgie der Eisenlegierungen, Technischer Buchverlag, Budapest, 1966, pages 282 to 299; or the text book by Dr. Gillemot, Die Technologie der Konstruktions-Werkstoffe, Vol. I, Lehrbuch-Verlag, Budapest, 1954).
According to operational experience, drive pulleys produced in accordance with the above technology cannot, even with very gentle operating condition, achieve an acceptable service life, since the far too rapid wear of the cast iron cable grooves by the steel cable obliges premature cable exchange. However, the cost outlay for maintaining the installation is thereby considerably increased, quite apart from the inevitably occurring standstill costs of the elevators, which, particularly in the case of multi-storey residential buildings, produce for the residents inconveniences, losses in time and nuisances which are difficult to tolerate. For the operators of such installations it would be acceptable if the service life of the combination of drive pulley and steel cable would allow at least a use cycle of approximately 10 years. Unfortunately, this desirable parameter value currently cannot even be approximately achieved with drive pulleys of lamellar graphite cast iron.
In drive pulleys, the maximum permissible groove pressure is dependent on the hardness of the groove material. An increase in the usual material hardness of the cable groove, which is made from lamellar graphite as an iron casting, of HB=180 kp/mm2 would also therefore be desirable, since according to currently applicable principles of drive pulley dimensioning the cable is to be assumed to be a continuous surface, but the groove pressure a uniformly distributed load. However, that does not entirely correspond to the actual position, since the loading of the steel cable is not at all to be considered “uniformly distributed”, so that “heart stresses” arise at the contact points (of the cable groove and steel cable).
In addition, wear of the drive pulley groove, with otherwise correct design, production, installation and operation in accordance with specifications, is decisively dependent on cable sliding or cable slippage that occurs. The relative speed of the sliding or slippage between cable and groove could, in principle, be prevented by an increase in drive capability, but this imposes a limit on the maximum permissible groove pressure value which in turn is a function of the drive pulley hardness.
A method of producing drive pulleys of elevator installations is known from Patent Specification EP 0 279 896, wherein the outer surface of the drive pulley, at least the cable-guiding groove surfaces, is subjected to a hardening process.
According to a feature of this patent specification, it is of advantage with greater loadings if after the processing of the cable-receiving groove surfaces to finished state these are subjected to a hardening process, preferably a flame process. It is thanks to this measure that, for example, with drive pulleys of the same dimensions and the same casting technology, cable grooves—which are appropriate to the different loading—of different surface hardness can be formed, but still with a relatively low cost outlay.
The hardening of the drive pulley described in Patent Specification EP 0 279 896 increases traction, but the material can embrittle and encourage formation of fractures. These irregularities lead to a shortened service life of the drive pulley. Thus, although the wear resistance of the drive pulley is increased by the hardening, the service life of the drive pulley is, on the other hand, shortened by the propagation of fractures.